1. Field of the Invention
The present invention relates to a drive circuit for an electroluminescence (hereinafter referred to as EL) display apparatus comprising an electroluminescence device and thin-film transistors (hereinafter referred to as TFT).
2. Description of the Prior Art
In recent years, EL display apparatuses using EL devices have gained attention as display apparatuses to replace CRTs and LCDS.
Furthermore, display apparatuses using TFTs as the switching devices for driving the EL device are being researched and developed.
FIG. 1 shows a circuit diagram of an organic EL display apparatus of the prior art.
According to the same diagram, a display pixel 1 of the organic EL display apparatus of the prior art comprises a first TFT 100, a second TFT 200, a holding capacitor 300, and an organic EL device 400.
A gate signal line G, which supplies a gate signal, and a drain signal line D, which supplies a drain signal, cross, and in the vicinity of the intersection of both signal lines G and D there are provided the organic EL device 400 and the TFTs 100, 200 for driving the organic EL device 400.
First, the first TFT 100 comprises a gate electrode 110, which is connected to the gate signal line G and supplied with the gate signal, a drain electrode 120, which is connected to the drain signal line D and supplied with the drain signal, and a source electrode 130, which is connected to a gate electrode 210 of the second TFT 200 and to the holding capacitor 300.
Next, the second TFT 200 comprises the gate electrode 210, which is connected to the source electrode 130 of the first TFT 100, a source electrode 220, which is connected to an anode 410 of the organic EL device 400, and a drain electrode 230, which is connected to a driving power supply 500 for supplying power to the organic EL device 400 so as to drive the organic EL device 400.
Furthermore, the organic EL device 400 comprises the anode 410, which is connected to the source electrode 220 of the second TFT 200, a cathode 420, which is connected to a common power supply terminal 600, and a light emitting device layer 430, which is sandwiched between the anode 410 and the cathode 420.
When the gate signal from the gate signal line G is supplied to the gate electrode 110 of the first TFT 100, the first TFT 100 turns on and the drain signal that was supplied from the drain signal line D is applied to the gate electrode 210 of the second TFT 200 and to the holding capacitor 300. As a result, the second TFT 200 turns on and a current flows, corresponding to the gate voltage of the second TFT 200, from the driving power supply 500 to the organic EL device 400 so that the light emitting device layer 430 of the organic EL device 400 emits light.
The organic EL device 400 is deposited in a sequence of the anode 410 formed from a transparent electrode, such as indium tin oxide (ITO), a first hole transport layer formed from 4,4'-bis(3-methylphenylphenylamino)biphenyl (MTDATA), a second hole transport layer formed from 4,4',4"-tris(3-methylphenylphenylamino)triphenylanine (TPD), a light emitting layer formed from 10-benzo[h]beryllium-benzoquinolinol complex (Bebq.sub.2) including a Quinacridone derivative, the light emitting device layer 430 formed from various electron transport layers formed from Bebq.sub.2, and the cathode 420 formed from a magnesium-indium alloy.
In the organic EL device, holes injected from the anode and electrons injected from the cathode are recombined within the light emitting layer so as to excite the organic molecules forming the light emitting layer and generate an exciton. In the process where the exciton deactivates, light is released from the light emitting layer. This release of light to the outside from the transparent anode through the transparent insulating substrate results in light being emitted.
On the other hand, it is necessary for the EL device in each display pixel to emit the same quantity of light so that a uniform and stable display is obtained at the surface of the EL display apparatus. However, since the characteristic of each second TFT 200 that is provided in each display pixel is not uniform, the currents supplied to the EL devices by the drive circuit for the EL display apparatus in the prior art cannot be kept uniform, thus resulting in a problem where the non-uniform currents appear as an uneven display among the display pixels.
Namely, the size of each second TFT varies, due to deviations in mask patterns during the manufacture of the TFTs and so forth, so that the current flowing to each drain varies even though the same gate voltage is applied to each second TFT. Therefore, the current supplied to the EL device differs with each display pixel and appears as an uneven display.